Method for making silicon-on-sapphire transducers

ABSTRACT

An improved method for making silicon-on-sapphire transducers including the steps of: forming a first silicon layer on a first side of a first sapphire wafer; bonding a second sapphire wafer to the first side of the first sapphire wafer such that the first silicon layer is interposed between the first and second sapphire wafers; reducing the thickness of the first sapphire wafer to a predetermined thickness; depositing a second silicon layer on a second surface of the first sapphire wafer, wherein the second surface of the first sapphire wafer is oppositely disposed from the first surface of the first sapphire wafer; bonding a silicon wafer to the second surface of the first sapphire wafer such that the second silicon layer is interposed between the first sapphire wafer and the silicon wafer, wherein the silicon wafer includes p+ regions indicative of a transducer structure and non-p+ regions; and, removing the non-p+ regions of the silicon wafer thus forming the transducer structure of p+ regions on the second surface of the first sapphire wafer.

FIELD OF INVENTION

The present invention relates to silicon-on-sapphire transducers, andmore particularly to an improved method for making the same.

BACKGROUND OF INVENTION

As is known, as a deflecting diaphragm, single-crystal sapphire hascertain unique advantages. Not only is it a single crystal that displaysno mechanical hysteresis when deflected, exhibiting only elasticdeformation, but is ultra-resistant to almost any chemical attack oretching. While this may be an advantage in a finished transducer, itcauses significant difficulties in fabrication.

Sapphire has a modulus of about 70×10⁶ PSI as compared to that ofsilicon which, in the transverse direction in the surface plane <100>(as defined on FIG. 4A), is about 20×10⁶ PSI. As a result, for the sameelectrical output from a piezoresistive Wheatstone bridge silicon grownor otherwise fastened to sapphire, the flexing diaphragm must be thinnerby the ratio of $\begin{matrix}\sqrt{\frac{{YaL}_{2}O_{3}}{Ysi}} & (1)\end{matrix}$

This means that to fabricate a pressure transducer of sapphire with thesame surface stress as one of silicon, since the surface stress isproportional to $\begin{matrix}{\frac{1}{Y}\quad \frac{a^{2}}{t^{2}}} & (2)\end{matrix}$

where a is the radius of the deflecting portion and t is the thickness,a diaphragm of sapphire must have a much larger diameter than one ofsilicon, or be much thinner.

However, because of the inert nature of sapphire, it is almostimpossible to thin the sapphire diaphragm by conventional means.Further, making a diaphragm of sufficient size to get enough stresslowers the number of sensors that can be made from an individual sliceas well as lowering the resultant natural frequency of the finishedsensors. Additionally, slices of commercially available sapphire areusually thick (about 0.020″). However, for a relatively small diametersensor, a thickness on the order of 0.005 inches is required.

The present invention is designed to overcome these constraints and toproduce a relatively smaller silicon on sapphire sensor with enhancedcharacteristics.

SUMMARY OF INVENTION

An improved method for making silicon-on-sapphire transducers includingthe steps of: sputtering or otherwise growing a silicon layer on a firstsurface of a commercially available first sapphire wafer.

Affixing a second sapphire wafer including a series of apertures to thefirst surface the first sapphire wafer, preferably by means of E.S.bonding, fusion bonding or any other suitable means as is understood bythose possessing ordinary skill in the pertinent art.

Lapping and polishing a second surface of the first sapphire wafer untilthe first sapphire wafer is reduced to a desired thickness. The secondsapphire wafer, which was previously secured to the first wafer, servesto strengthen the first wafer during this process.

Growing and/or sputtering a silicon layer on the now polished surface ofthe first sapphire wafer. This silicon layer is then oxidized using anywell known, conventional technique.

A third wafer containing a series of sensor networks is formed using anysuitable process. The third wafer includes p+ areas which include asensor network and a group of contact areas extending from the sensornetwork. Outside of the contact areas, another p+ area separate from thecontact areas but surrounding them, is also formed. The third wafer isthen preferably fusion-bonded to the oxidized surface of the firstsapphire wafer, and the non p+ areas are removed preferably by using aconductivity selective etch.

Appropriate areas in the p+ contact regions are preferably metalizedusing a high temperature metal system such as platinum silicide,titanium, or platinum.

A fourth wafer made of glass or any suitable, non-conductive material,is prepared such that there are a series of apertures corresponding tothe various metalized contact areas on the first wafer. Additionally,there is preferably provided a slight depression in the fourth wafercorresponding to the area of the sensor network to allow the diaphragmto deflect without touching the glass.

Finally, the glass wafer is preferably sealed using an ES Bond to the p+contact areas, and the apertures over the metalized areas may be filledwith a glass metal frit to make electrical contacts to metalized areasof the p+ regions. If the seal of the glass wafer to the p+ contact areais not through a central aperture in the glass wafer over the deflectingportion of the first sapphire wafer, an absolute sensor will result. Ifthere is a through-central aperture, a gage or differential sensor willresult.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a cross section of a silicon and sapphire structureaccording to the present invention.

FIG. 2 illustrates a cross section of a structure according to FIG. 1having an additional sapphire wafer bonded according to the presentinvention.

FIG. 3 illustrates a cross section of a structure according to FIG. 2after thinning according to the present invention.

FIG. 4A illustrates an isometric view of a silicon wafer utilizedaccording to the present invention.

FIG. 4B illustrates a cross section of the silicon wafer of FIG. 4A.

FIG. 5 illustrates a cross section of a silicon-on-sapphire structureaccording to the invention.

FIG. 6 illustrates a perspective view of a seal being applied to thestructure of FIG. 5.

FIG. 7 illustrates an enlarged cross section of a structure formedaccording to method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the many figures, wherein like references refer to likeelements of the invention, FIG. 1 illustrates a commercially availableslice of sapphire 10 having a thin silicon layer 20 grown on one surface13, for example by sputtering.

FIG. 2 illustrates a second sapphire wafer 30 including a series ofapertures 40 each having a diameter D which is preferably about 0.150inches in diameter, affixed to surface 13 of the first wafer by means ofE.S. bonding, fusion bonding or any other suitable means as isunderstood by those possessing ordinary skill in the pertinent art. Anexample of E.S. bonding can be seen in commonly assigned U.S. Pat. No.3,951,707, entitled Method for Fabricating Glass-backed Transducers andGlass-backed Structures, the disclosure of which is incorporated byreference herein, while a suitable example of fusion bonding isillustrated in commonly assigned U.S. Pat. No. 5,286,671, entitledFusion Bonding Technique for Use in Fabricating Semiconductor Devices,the disclosure of which is also incorporated by reference herein.

The apertures 40 can be formed prior to bonding sapphire wafer 30 tosapphire wafer 10 using any suitable means known to those skilled in theart.

Referring now to FIG. 3, therein is illustrated a next step according tothe present invention, wherein the surface 17 (opposing surface 13) ofthe sapphire wafer 10 is then lapped and polished until the firstsapphire wafer 10 is reduced to a desired thickness, preferablyapproximately 0.005 inches. The second wafer 30, which was previouslysecured to the first wafer 10, serves to strengthen the first wafer 10during this process.

Next, a silicon layer 50 is grown, for example sputtered, on the nowpolished surface 17 of the first wafer 10. This silicon layer 50 is thenpreferably oxidized using any well known, conventional technique.

Referring now to FIGS. 4A and 4B, a silicon wafer 60 containing a seriesof sensor networks 70 is formed using any suitable process. The siliconwafer 60 includes p+ areas 80 which include sensor network 70 and agroup of contact areas 90 extending from the sensor network 70. Outsideof the contact areas 90, another p+ area 100 separate from the contactareas 90 but surrounding them, is also formed. The silicon wafer 60 ispreferably fusion-bonded to the oxidized surface 17 of the first wafer10, using the techniques described in the previously incorporated,commonly assigned U.S. Pat. No. 5,286,671, and the non p+ areas areremoved using any suitable method, for example a conductivity selectiveetch, resulting in a structure such as that illustrated in FIG. 5. Theremaining structure can be seen to include the p+ portions 80 of thesilicon wafer 60 fusion bonded to the oxidized surface 50 of the firstsapphire wafer 10, for example.

Next, appropriate areas 110 in the p+ contact regions 90 are preferablymetalized using a high temperature metal system such as platinumsilicide, titanium, or platinum.

Referring now to FIG. 6, a fourth wafer 120 made of any suitable,non-conductive material, such as glass for example, is prepared suchthat there are a series of apertures 130 corresponding to the variousmetalized contact areas 110. Additionally, there is preferably provideda slight depression 150 in the glass wafer 120 corresponding to the areaof the sensor network 70 to allow the diaphragm to deflect withouttouching the glass wafer 120.

Referring now also to FIG. 7, the glass wafer 120 is preferably sealedusing an ES Bond to the p+ contact areas 90, and the apertures 130 overthe metalized areas 110 are filled with a glass metal frit 140 to makeelectrical contacts to metalized areas 110 of the p+ regions 80. If theseal of the glass wafer 120 to the p+ contact area 90 is not through acentral aperture in the glass wafer 120 over the deflecting portion ofthe first wafer 10, an absolute sensor will result. If there is athrough-central aperture, a gage or differential sensor will result.

There are a number of unanticipated advantages to the structure above.The method makes possible the construction of relatively smallsilicon-on-sapphire transducers with greater sensitivity and higherfrequency response at a lower cost than conventional methods. The methodmakes possible the construction of either gauge or absolute transducerswithout the need for additional hermetic seals with headers, etc. as isunderstood by those possessing ordinary skill in the pertinent art. Themethod eliminates the need for ball-bond contacts to thesilicon-on-sapphire surface resulting in greater reliability. The methodmakes possible much higher temperature operation by using a metal-glassfrit as the contact means to the p+ region. The method does not requireepitoxial growth of silicon-on-sapphire making it cheaper. And, the useof fusion bonding enables a much more degenerate sensing network to beformed.

Having described the preferred embodiment of this invention, it isevident that other embodiments incorporating these concepts may be used.Accordingly, although the invention has been described and pictured in apreferred form with a certain degree of particularity, it is understoodthat the present disclosure of the preferred form has been made only byway of example and that numerous changes in the detail of construction,in combination, and arrangement of parts, may be made without departingfrom the spirit and scope of the invention as here and after claimed. Itis intended that the patent shall cover by suitable expression in theappended claims, whatever features of patentable novelty exist in theinvention disclosed.

We claim:
 1. An improved method for making silicon-on-sapphiretransducers comprising the steps of: forming a first silicon layer on afirst side of a first sapphire wafer; bonding a second sapphire wafer tosaid first side of said first sapphire wafer such that said firstsilicon layer is interposed between said first and second sapphirewafers; reducing the thickness of said first sapphire wafer to apredetermined thickness; depositing a second silicon layer on a secondsurface of said first sapphire wafer, wherein said second surface ofsaid first sapphire wafer is oppositely disposed from said first surfaceof said first sapphire wafer; bonding a silicon wafer to said secondsurface of said first sapphire wafer such that said second silicon layeris interposed between said first sapphire wafer and said silicon wafer,wherein said silicon wafer includes p+ regions indicative of atransducer structure and non-p+ regions; and, removing said non-p+regions of said silicon wafer thus forming said transducer structure ofp+ regions on said second surface of said first sapphire wafer.
 2. Themethod of claim 1, wherein said second sapphire wafer comprises aplurality of apertures.
 3. The method of claim 2, wherein each of saidplurality of apertures is approximately 0.150 inches in diameter.
 4. Themethod of claim 1, wherein said second sapphire wafer is E.S. bonded tosaid first sapphire wafer.
 5. The method of claim 1, wherein said secondsapphire wafer is fusion bonded to said first sapphire wafer.
 6. Themethod of claim 1, wherein said step of reducing the thickness of saidfirst sapphire wafer comprises the step of lapping, or polishing saidsecond surface of said first sapphire wafer until said first sapphirewafer is approximately of said predetermined thickness.
 7. The method ofclaim 6, wherein said predetermined thickness is approximately 0.005inches.
 8. The method of claim 1, wherein said depositing of said secondsilicon layer comprises sputtering.
 9. The method of claim 1, whereinsaid depositing of said second silicon layer comprises growing saidsecond silicon layer.
 10. The method of claim 1, further comprising thestep of oxidizing said second silicon layer.
 11. The method of claim 10,wherein said silicon wafer is fusion bonded to said oxidized secondsurface of said first sapphire wafer.
 12. The method of claim 1, furthercomprising the step of sealing a non-conductive wafer including adepression in a surface thereof to said second surface of said firstsapphire wafer over said p+ regions.
 13. The method of claim 12, whereinsaid step of sealing comprises the step of E.S. bonding.
 14. The methodof claim 13, further comprising the step of forming a plurality ofapertures in said non-conductive wafer at given locations correspondingto electrical contact locations.
 15. The method of claim 14, whereinsaid non-conductive wafer comprises glass.
 16. The method of claim 15,further comprising the step of filling each of said plurality ofapertures in said non-conductive wafer with a glass metal frit.
 17. Themethod of claim 1, wherein said silicon wafer comprises a sensor networkand a plurality of contact areas, and said method further comprises thestep of selectively etching those portions of said silicon wafer notcorresponding to said sensor network or any of said contact areas. 18.An improved method for making silicon-on-sapphire semiconductorstructures comprising the steps of: forming a first silicon layer on afirst side of a first sapphire wafer; bonding a second sapphire wafer tosaid first side of said first sapphire wafer such that said firstsilicon layer is interposed between said first and second sapphirewafers; reducing the thickness of said first sapphire wafer to apredetermined thickness; depositing a second silicon layer on a secondsurface of said first sapphire wafer, wherein said second surface ofsaid sapphire wafer is oppositely disposed from said first surface ofsaid first sapphire wafer; bonding a silicon wafer to said secondsurface of said first sapphire wafer such that said second silicon layeris interposed between said first sapphire wafer and said silicon wafer,wherein said silicon wafer includes regions of a first conductivity typeindicative of a transducer structure and regions of a secondconductivity type; and, removing said regions of said secondconductivity type thus forming said semiconductor structure of saidregions of said first conductivity type on said second surface of saidfirst sapphire wafer.